OBSERVATIONS ON POLYZOA. 207 



The arms of the polypides of Plumatella separate at 

 a much earlier date than those of Pectinatella ; and the 

 latter, probably, earlier than the arms of Cristatella. Be 

 this as it may, the approximation of the tips of the arms 

 is an embryonic character; because, as already stated 

 (remarks on reproduction) , the arms in the young divide 

 from within outward, and the tips are the last to be sep- 

 arated, and the longer the arms, the longer the time nec- 

 essary to complete the division. It seems likely, there- 

 fore, that the embryonic character of the arms in Crista- 

 tella is due entirely to their increased length. 



TENTACLES. 



The number of the tentacles are increased in the indi- 

 vidual, as has been shown, by additions at the tips of the 

 arms as the latter increase in length ; therefore, the longer 

 the arms, the greater the number of tentacles. This law 

 is carried out, not only in the diiferent individuals of the 

 species, but it also determines the increment in the num- 

 ber of tentacles from Fredericella, which never has over 

 twenty-four, to Cristatella, which, as far as we know, never 

 has over ninety. The length of the tentacles, however, 

 decreases ; and the decrement is due, not to any decrease 

 in the tentacles themselves,, but to the comparatively 

 greater increase in the length of the arms. 



CALYX. 



The breadth of the calyx appears to be a fluctuating 

 character. 



EPISTOME. 



The form of the base is an incremental character, the 

 change from an elliptical to a crescent shape taking place 

 when the arms appear in Plumatella. 



TENTACULAR MUSCLES. 



The number of pairs of these must be equal to the 

 number of tentacles on any one lophophore ; and since 

 the number of tentacles is determined by the length of 



